The present invention relates to methods for determining the presence in water of dispersed liquids of low solubility. More particularly, the invention relates to methods for quantitative analysis of very low concentrations of particulate oil in water.
Pollution control laws have led to an urgent need for a sensitive, rapid and simple method for measuring very low concentrations of oil in the water. For example, the Federal Water Pollution Control Act prohibits oil in waste water streams from ships within the off-shore 12 mile limit of the United States if such discharge causes a visible sheen upon the surface of the water. In the case of a Coast Guard Cutter, such a sheen is produced when the oil content of the water pumped overboard reaches between 20 and 30 ppm. At certain Naval installations, the maximum allowable oil and grease concentration in water discharges is only 5 ppm. Therefore, to be effective, a method for monitoring oil in waste water streams should be sensitive within a range of 0 to 30 ppm at 5 ppm minimum increments. Most currently available laboratory and automatic oil detection methods involve gravimetric or volumetric measurement after extracting the oil with volatile solvents or by centrifugal separation, or measurement of light absorption, scattering, or fluorescence. In general, such methods are not suitable for shipboard use because they are too time-consuming, expensive, insensitive, or require the services of a skilled analyst. Moreover, the accuracies of many of the prior art methods are greatly reduced by surfactants and solid contaminents often present in the bilge water of ships.